I usually preface this kind of post with the declaration that my father was a butcher. He slaughtered, aged, cut, displayed, wrapped meat for over 60 years. What I'm writing is based on my time working with him and his advice. Here's what he thought–
• Beef graders are human. They make mistakes. They may grade a side of beef too high or too low.
• The grade is established for both sides by always cutting between the same two ribs and examining the marbling and such. It's possible that other parts of the animal may not grade the same as the part examined.
• Prime grade is not worth the money. He always recommended what he called, "A high grade of Choice."
BTW, when he slaughtered for the family he always selected an Angus heifer.